This disclosure is directed to a formation testing procedure which measures formation pressures over a period of time and particularly formation pressures for formations behind a cased well borehole. After a well has been drilled and it has been determined that some formation of interest will produce in quantity, the hole is typically cased and perforations are formed through the casing into one or more formations to produce oil or gas, sometimes with a mixture of water or sand. Production continues for an interval after which formation pressures typically start to drop. Often, a formation is capable of producing by formation pressure drive. As formation pressures drop, the formation may be produced by placing various types of pumping devices in the borehole. Ultimately, formation pressure will drop and subsequent remedial or secondary completion techniques are used. An important factor is the formation pressure and particularly formation pressure change over a period of time and especially after the formation has been partially depleted. While one formation may be depleted completely, another formation isolated from the well by the casing can be completed long after the casing has been installed. In these and other circumstances, it is appropriate to go into the well with a formation pressure test tool, sometimes known as a formation tester, and perform subsequent tests of the formation to obtain data regarding either the produced formation or other formations.
An important data is the rate of formation pressure change over a period of time. Typically, a formation tester is connected with the formation of interest and time decay pressure measurements are taken. This involves forming a small perforation through the casing into the formation. For this purpose, the formation tester normally includes an extendable pressure pad which is mounted on an extendable test probe. The pressure pad is brought firmly to contact the casing and contours against the casing to prevent leakage around the pressure seal encircling the tip of the test probe. It is forced against the casing while a backup shoe on the opposite side of the formation tester is extended to hold the formation tester in location. A small shaped charge is detonated to form a small hole (perhaps one centimeter in diameter) through the casing and into the formation.
The present disclosure sets forth methods and an apparatus which carries out the foregoing tests and several additional tests as will be described. Consider, as an example, one advantageous test. Assume a field having several wells, and further assume that a particular well is to be used as an injection well to practice secondary recovery techniques featuring injection of one fluid into one well with the hope that enhanced recovery at nearby adjacent wells will be observed. In the past, an assumption has been made, in the absence of contrary data, that fluid flow from the formation occurs at the same rate at which fluid can flow back into the formation. Assume that a particular formation produces a specified volume of fluid in a twenty-four hour period. Assume further that this flow for one day produces a formation pressure drop of 3 psi. It has been assumed that injection back into that particular formation of the same fluid volume over one day will in similar fashion raise the formation pressure by about 3 psi. In general, the formation has been treated as a type of bidirectional conduit having a known or measurable resistance to fluid flow. This is not necessarily true, and it appears to be more untrue especially for unconsolidated formations and diatomite formations. Assume that the perforation through the casing opens into an unconsolidated formation. If flow is from the formation into the cased well, production of formation sand will occur. This permits some shifting and will ultimately change formation pressure while also locally changing formation porosity. By contrast, if the same quantity of fluid is injected back into the formation without the sand that was previously produced, there is no precise relationship which states what the formation pressure should be at the completion of reinjection of the same quantity of fluid. The formation does not permit fluid flow bidirectionally. Accordingly, if several wells in a common field are unitized for secondary recovery, and certain of the wells are converted into injection wells while the remainder of the wells are recovery wells, the assumption that flow is easily established from the injection well to the nearby recovery wells is erroneous.
In summary, the present disclosure sets forth a method and apparatus which enables formations to be pressure tested in a different fashion and in particularly in a cased borehole.